1. Field of the Invention
The present invention relates to a method of making doped semiconductors, to doped semiconductors and to products made thereof.
In another aspect present invention relates to a method of making doped silicon, to doped silicon and to products made thereof.
In even another aspect, the present invention relates to a method of making solar cells, to solar cells and to products including such solar cells.
2. Description of the Related Art
Solar cells as a potential source of energy were discovered and developed in the 1950""s at Bell Labs.
For certain applications, solar cells are viewed as the only practical power source. For example, for orbiting space satellites, with payload weight for fuel at a premium, and refueling a very difficult and costly proposition, solar cells serve to power the satellite through its orbit. As another example, unmanned exploratory space probes, which lack not only suitable payload space for fuel, but which cannot be refueled are powered by solar cells.
For terrestrial applications, solar power has long been hailed as an ideal fuel source. Solar power is environmentally clean and appears, in comparison to human lifespans and needs, to be an infinitely renewable power source. Some studies assert that less than 0.3% of the surface of earth could be covered with solar cells to meet all the energy needs of the world.
It has long been understood that for solar power to become used not only for specialized uses, but for common every day applications, it must become economic. During the late 1970""s and early 1980""s with ever escalating fossil fuel prices, it was believed that solar power would be economical by the 1990""s. However, lack of noticeable advancements in solar energy technology, combined with stabilization of fossil fuel prices, has not been helpful in promoting the use of solar power.
At present, solar power provides a mere fraction (less than about 0.01%) of the current power usage of the whole world. The general trend appears to be in the direction of increased solar power usage. The cost of solar generated energy has been steadily going down since 1960""s as its production has been going up. Even at the current average price of near $3 per peak watt there exists a large market such as in remote telecommunication repeaters, fibre optic amplifiers, remote street signs, telephone booths along highways, or lights, and remote homes and cabins. Further reduction in cost will only serve to open up the market for utility scale power generation.
The first amorphous Si:H solar cell was produced in 1976 at the RCA laboratories by Carlson and Wronsky. Immediately after its discovery, the a-Si:H solar cell made the transition from laboratory research to commercialization due to its cost effectiveness. While these a-Si:H solar cells are now being used in many solar energy operated electronic products, low efficiency, degradation during long usage, and other deficiencies have prevented it from becoming a viable power source in many applications.
Polycrystalline and single crystalline silicon technology currently holds about 82% of the world wide solar cell market. In 1995 this amounted to about 67 MWatts of power generated which translates to a 200 million dollar market. Photovoltaic power generation is increasing at a phenomenal rate and it is projected that by the year 2010, about 10% of the utility bulk power will be derived from the photovoltaic solar cells. Making these crystalline silicon solar cells at low cost will push its insertion into the power market even faster. The current crystalline/multi-crystalline silicon solar cell fabrication technology involves diffusion steps which require temperatures as high as 1100xc2x0 C. and metallization steps which require sintering temperatures as high as 600xc2x0 C. Most of the crystalline silicon solar cells in the market are made of multi-crystalline silicon substrates as they are much cheaper and available in larger sizes. Unfortunately, high temperature diffusion causes diffusion spikes along the grain boundaries in multi-crystalline silicon solar cells possibly causing device degradation and failure. Additionally, with larger substrate sizes, it is extremely difficult to maintain relatively uniform diffusion across the substrate.
Current fabrication technology also requires a separate anti-reflection coating to reduce reflection losses and increase efficiency. Current fabrication technology further requires a step to create a rough surface texture to reduce reflection losses and increase light trapping. This mechanical texturing step increases fabrication costs.
Finally, current fabrication technology also requires a hydrogen passivation step, generally hydrogen plasma passivation, to passivate the grain boundries in multi-crystalline silicon solar cells.
However, in spite of the many advancements in the prior art, the prior art suffers from the disadvantages as detailed above.
Thus, there is still a need for an improved method of making solar cells, and for improved solar cells made therefrom.
There is another need in the art for a method of making solar cells which does not require high temperature diffusion steps or metallization steps, and for improved solar cells made therefrom.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
It is an object of the present invention to provide for an improved method of making solar cells, and for improved solar cells made therefrom.
It is another object of the present invention to provide for a method of making solar cells which does not require high temperature diffusion steps or metallization steps, and for improved solar cells made therefrom.
These and other objects of the present invention will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
According to one embodiment of the present invention, there is provided a method of forming a polycrystalline semiconductor layer on a substrate. The method generally includes depositing a semiconductor onto the substrate to form an amorphous semiconductor layer. The method further includes depositing a metal onto the semiconductor layer to form a structure comprising the substrate, amorphous semiconductor layer and a doping metal layer. The method even further includes annealing the structure at a temperature in the range of about 170xc2x0 C. to about 600xc2x0 C. to convert at least a portion of the amorphous semiconductor layer into polycrystalline. Suitable examples of semiconductor include silicon, germanium, silicon-germanium alloys, germanium-carbon alloys, silicon-carbon alloys, and silicon-nitrogen alloys
According to another embodiment of the present invention, there is provided metal doped silicon. The doped silicon generally includes polycrystalline silicon comprising greater than about 1xc3x971020 dopant atoms per cm3 of silicon.